Malaria and dengue vector biology and control in Southeast Asia

10
Malaria and dengue vector biology and control in Southeast
Asia
Pattamaporn Kittayapong#
Abstract
This chapter reviews the situation of vector biology and control of both malaria
and dengue in the Southeast-Asian region as part of the World Health Organization
(WHO/TDR) working-group meeting on strategic planning to bridge laboratory and
field research in disease vector control. Many research studies on malaria were related
to the survey of malaria vectors and parasites and their spatial and temporal
distribution in each country. A few studies demonstrated application of molecular
tools to identify sibling species in the vector complexes as well as the genetic
structure and gene flow among these complex species. Despite insecticide resistance
having been detected in many vector species, insecticide-impregnated bednets are still
reported as a cost-effective and efficient way for malaria control. Social-science and
socio-economic studies indicate that the level of education and poverty is related to
the risk of malaria infection and also emphasize the importance of education as part of
successful control programmes. The majority of research on dengue vectors in
Southeast Asia involves surveillance for species composition, relative abundance and
seasonal distribution of both immature and adult stages. Identification of key breeding
containers and patterns of landing/biting of adults are routinely investigated in the
study areas. Some aspects of vector ecology and vector biology related to the
symbionts of the mosquitoes have been reported. Several studies have pointed out the
importance of human transportation as a means for spreading dengue. Recent studies
also demonstrated that the disease spread from the larger cities, which serve as the
viral reservoirs, to smaller communities in a radial manner. Several socio-economic
studies in different countries indicate variations in knowledge and practice related to
dengue. Dengue control programmes in Southeast Asia have recently shifted from
application of insecticides to integrated vector control strategies using biological
control agents, pyrethroid-based insecticides, source reduction and environmental
management. However, most of the present vector control measures are not
sustainable due to several factors related to both community participation and
persistence of public-health vector control programmes. Genetic control using modern
molecular technologies may offer novel solutions for future control of vector-borne
diseases.
Keywords: Aedes; Anopheles; dengue; malaria; mosquito; vector; Southeast Asia
#
Center for Vectors and Vector-Borne Diseases and Department of Biology, Faculty of Science,
Mahidol University, Rama 6 Road, Bangkok 10400, Thailand. E-mail: grpkt@mahidol.ac.th
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Malaria
Spatial distribution of malaria vectors and parasites
Malaria remains an important health threat in rural areas of Southeast Asia. All
four known human malaria parasites, but predominantly Plasmodium falciparum and
P. vivax are present in the region. Members of three species complexes of Anopheles
mosquitoes, An. dirus, An. maculatus and An. minimus, are the most important
vectors. In Vietnam, malaria is found in mountainous and woody areas as well as in
coastal regions. The main vector is An. dirus, which is found in stagnant and shaded
waters in forested areas, whereas An. minimus, another vector species, breeds in
running streams in hilly areas. In contrast, An. sundaicus, a coastal vector species,
adjusts to a variety of habitats. P. falciparum and P. vivax occur at about the same rate
except for the woody regions where P. falciparum is more prevalent (75%) (Nguyen
1993).
A preliminary survey of Anopheles in 8 provinces in Laos showed that out of 19
species collected, An. aconitus is the predominant species, especially in the month of
December, and only 3 species, i.e. An. dirus, An. maculatus and An. minimus, are
infected with oocysts (Vythilingam et al. 2001). Another malaria survey in one of the
southeastern provinces of Laos has reported 28 species collected by both human and
animal baits. Sporozoites of both malaria parasites were found in An. dirus and An.
minimus as well as in An. philippinensis. Four species, An. notanandai, An.
sawadwongporni, An. willmori and An. Hodgkini, were recently reported for the first
time in this region (Toma et al. 2002). A field survey for malaria prevalence in
southeastern Laos by PCR assay has shown that the most common malaria parasite is
P. falciparum, and mixed infection by 2-4 species of parasites was detected in 23.1%
of the samples (Toma et al. 2001). Interestingly, there is a report that P. falciparum,
but not P. vivax, is associated with acute malnutrition among youths in Laos
(Takakura et al. 2001).
In Myanmar, An. dirus is reported to be one of the primary vectors of P.
falciparum causing cerebral malaria. The positive rate during 1998-2000 in Bago,
Mandalay and Tanintharyi Divisions ranged from 9.9% to 34.3% (Oo, Storch and
Becker 2003). A recent entomological survey conducted in Vietnam, Laos and
Cambodia showed that An. dirus A is still an important malaria vector despite its low
density, whereas the role of An. minimus A in malaria transmission varies both
temporally and spatially. The brackish-water-breeding species, An. sundaicus occurs
in high density due to the recent changing patterns of land use from rice cultivation to
shrimp farming (Trung et al. 2004).
In peninsular Malaysia, An. maculatus is the main vector of malaria even though
its abundance is only 9.1%, which is about half of the average number of the other
two dominant species, An. aconitus and An. barbirostris (Rahman, Adanan and Abu
Hassan 2002). In Indonesia, malaria is common throughout the country and P. vivax is
the most abundant malaria parasite (Rodhain 2000). The primary vector species,
especially in forested hilly areas of Java, are An. maculatus and An. balabacensis
(Barcus et al. 2002).
Analysis of the distribution of malaria-endemic areas in the Indochina peninsula
has been conducted using GIS/remote-sensing techniques. These are useful for
identification of endemic malaria based on the normalized difference vegetation
indices (NDVI) (Nihei et al. 2002). GIS analysis was also used to study distribution of
the larval stages of An. flavirostris, a principle malaria vector in the Philippines. The
study has shown that early larval instars are clustered in shady stream-bank areas,
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whereas the late instars are weakly related to shade (Foley, Torres and Mueller 2002).
The GIS-based spatial patterns of surface slope and wetness were used to identify the
distribution of breeding sites of four major malaria vectors, An. dirus, An. maculatus,
An. minimus and An. sawadwongporni in Northern Thailand (Sithiprasasna et al.
2003b). Spatial and temporal distribution of Anopheles mosquitoes in the same area
was reported. A total of 21 species were collected and 86% of species biting humans
were An. minimus, which was found to be infected with both P. falciparum and P.
vivax (Sithiprasasna et al. 2003a). Konchom et al. (2003) studied malaria incidence
during 1991-2001 in 30 highly endemic provinces along the Thai borders. They
reported the trend of malaria parasite species shifting from P. falciparum to P. vivax
along the western border to Myanmar and northern border to Laos as well as along the
eastern border to Cambodia, while the opposite trend of parasite distribution was
found in the southern border to Malaysia. There was also a significant difference in
annual parasite incidence between border and non-border especially along the border
with Myanmar and Cambodia. A survey on malaria among mobile Cambodians at the
Thai-Cambodia border reported an overall infection rate of 2.4% with 93.8% of the
infections being due to P. vivax (Kitvatanachai et al. 2003).
Population genetics and identification of Anopheles species complexes
Understanding population genetics of mosquito vectors is important in planning
malaria control. In Thailand, microsatellite markers have been developed for studying
genetic variations in natural populations of An. maculatus (Rongnoparut et al. 1996).
A large number of alleles and high polymorphisms have demonstrated the usefulness
of these microsatellite markers in studying gene flow and population-genetic structure
of this vector species. High levels of genetic diversity in a small population of An.
maculatus were detected in the above study. Population structure and population
history of An. dirus, the main vectors of malaria in Southeast Asia, were studied using
sequence analysis of the mitochondrial COI gene (Walton et al. 2000). This study
reported that An. dirus A extends eastward from Thailand to Laos, Cambodia and
Vietnam while An. dirus D extends westward through Myanmar. Both species are
parapatric but there is little genetic differentiation either within or between species.
Anopheles dirus C has a patchy distribution along the Thai-Myanmar border and also
extends southward into peninsular Thailand. However, no gene flow between
populations of An. dirus C has been detected. Because of greater genetic diversity in
species D, it has been hypothesized that population expansion occurred first in this
species and subsequently in species A.
In Southeast Asia, the presence of species complexes makes it more difficult to
identify the vector species correctly, which may lead to the wrong target in vector
control. In general, members of the sibling species usually exhibit behavioural
differences. Two species within the An. minimus complex, with differences in resting
and biting behaviours, have been discovered in Vietnam using isozyme
electrophoresis (Van Bortel et al. 1999). In central Vietnam, the misidentification of
An. minimus as An. varuna was a good example of the importance of the correct
identification of vector species in order to implement malaria control effectively. Both
Anopheles species are different in feeding behaviour, i.e., An. varuna is highly
zoophilic and is not considered to be a vector, whereas An. minimus feeds on both
animals and humans and has been confirmed as a vector (Van Bortel et al. 2001).
Recently, a multiplex PCR assay was developed to identify the members of the An.
minimus complex as well as other closely related species, i.e., An. aconitus, An.
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pampanai and An. varuna (Phuc et al. 2003). This technique can be applied to all life
stages and is simpler, quicker and cheaper than previous assays.
Insecticide resistance and behaviour of malaria vectors
Development of insecticide resistance among vector species in the Southeast-Asian
region is an important factor leading to failure in malaria control. Insecticide
resistance patterns in mosquito vectors in Thailand have been reported by
Chareonviriyaphap, Aum-aung and Ratanatham (1999) and Prapanthadara et al.
(2000). Behavioural responses of An. minimus to DDT, deltamethrin and
lambdacyhalothrin were evaluated using an excito-repellency escape chamber. Both
colony-reared and wild populations of An. minimus exhibited insecticide-avoidance
behaviour, i.e., contact irritancy and non-contact repellency (Chareonviriyaphap et al.
2001). The behavioural avoidance response to insecticides, which was the first sign of
insecticide resistance, was detected not only in An. minimus but also in three other
malaria vectors, i.e. An. dirus, An. maculatus form B and An. sawadwongporni,
regardless of insecticide susceptibility, age, nutritional or physiological status
(Chareonviriyaphap, Prabaripai and Bangs 2004). Later, the change of feeding
behaviour in natural populations of An. minimus A and C in responding to DDT
spraying was reported. Both species tend to feed on cows rather than humans and
there was no preference for indoor, outdoor or forest biting (Rwegoshora et al. 2002).
Recently, seasonal abundance and blood-feeding activity of An. minimus was studied
in Western Thailand (Chareonviriyaphap et al. 2003). Results indicated that this
species is more abundant during the wet season and that the human-biting peak in this
area is different from other areas suggesting site-specificity in feeding behaviour. This
study concluded that site-specific studies were necessary to evaluate vector behaviour
accurately as it relates to malaria transmission.
Malaria vector control
Malaria control programmes in Southeast Asia are quite difficult to accomplish due
to the presence of vector species complexes and insufficient information on the
feeding behaviour of vectors, as well as the resistance of vectors to insecticides.
Malaria vector control in this region has recently shifted from routine, residual space-
spraying inside houses to the use of pyrethroid-impregnated bednets. In Laos,
impregnated bednets have recently been reported to reduce malaria transmission
successfully (Kobayashi et al. 2004). Despite a successful reduction of malaria in
Thailand, re-emergence of the disease was evident by an increase of the annual
parasite indices during 1998 (Chareonviriyaphap, Bangs and Ratanatham 2000).
Evaluation of repellency and killing effects of bednets treated with etofenprox,
deltamethrin, lambdacyhalothrin and permethrin was carried out in Northern Thailand
and results showed that all four insecticides have a high repellence effect. However,
the problems of cross-resistance, persistence of chemicals and types of mosquito-net
materials should be considered for further evaluation (Prasittisuk et al. 1996).
An evaluation of malaria control in central Vietnam showed that both spraying of
insecticides in and around the houses and the use of insecticide-impregnated bednets
were efficient (Nguyen et al. 1996). However, spraying with lambdacyhalothrin was
more effective than with pyrimiphos and DDT. It was recently reported that malaria in
Vietnam, as well as in other Southeast-Asian countries, is related to forest activities so
control efforts should target forest workers. Experiences of malaria control in refugee
camps on the Pakistan-Afghanistan and Thailand-Myanmar borders had concluded
that both government and non-government agencies could play a significant role in
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solving issues in malaria control. Moreover, integration of research within
implementation programmes may result in innovation and sustainable malaria control
(Rowland and Nosten 2001).
Social sciences and socio-economics related to malaria
Social factors related to malaria occurrence have been studied in Eastern Thailand
(Butraporn, Sornmani and Hungsapruek 1986). This study demonstrated that poor
education and low income as well as long residency and frequent forest association
led to a high risk of malaria infection. A study on the behaviour dealing with self-
prevention of malaria among mobile populations in Eastern Thailand indicated that
the age group of 30-39 years old has the highest risk due to periodic movement into
the forested areas. Their moderate knowledge of, and attitude to malaria does not
enable them to protect themselves against it (Butraporn et al. 1995). A more recent
study in Thailand also indicated the importance of both socio-economic and cultural
factors affecting malaria control programmes (Panvisavas 2001; Panvisavas,
Dendoung and Dendoung 2001). In Laos, a study on knowledge and behaviour of
people regarding prevention of malaria, conducted in 1999-2000, showed that the
level of malaria prevention was related to the level of education (Uza et al. 2002).
Health education in the target community is, therefore, an important component for
the success of vector control programmes.
In Thailand, the use of impregnated bednets to prevent malaria in children is high
among mothers who have knowledge about the disease (Sri-aroon et al. 1998). The
cost-effectiveness of lambdacyhalothrin-treated bednets was evaluated against the use
of DDT spraying and malaria surveillance in Western Thailand. Results showed that
the bednet programme was most cost-effective when compared to DDT spraying and
malaria surveillance ($1.54 versus $1.87 and $2.50 per case of prevented malaria)
(Kamolratanakul et al. 2001). A pilot malaria control programme using DDT-
impregnated bednets in Laos during 1995 to 1997 was evaluated. The villages where
treatment occurred showed a significant increase of the number of bednets used and a
significant decrease of malaria infection when compared to control communities. In
addition, this study reported that risk factors were related to occupation, location of
the house and use of mosquito nets (Philavong et al. 2000).
Dengue and dengue hemorrhagic fever
Biology and ecology of dengue vectors
In Southeast Asia, both Aedes aegypti and Ae. albopictus are important vectors of
dengue (DF) and dengue hemorrhagic fever (DHF). Several studies on Aedes vectors
in this region have reported the distribution and abundance of both immature and
adult stages. In Sarawak, Indonesia, a survey for larvae of Ae. aegypti and Ae.
albopictus in urban housing indicated that both species shared habitats in houses (9%)
and in vacant land (4.5%) (Seng and Jute 1994). In South Sulawesi, Indonesia, Ae.
aegypti was found mainly in earthen jars indoors while Ae. albopictus bred mainly in
drum cans in hilly and mountainous areas (Ishak et al. 1997). In Thailand, the biology
of both dengue vectors on Samui Island was reported (Thavara et al. 2001). The larval
habitats of both species were distinctly separated, i.e., Ae. aegypti preferred to breed
in earthen jars and concrete water storages while Ae. albopictus bred in coconut husks
and coconut floral spathes that held rain water. Aedes aegypti eggs were not detected
in outdoor ovitraps at a distance of 1.5 meters from houses and 75.4% of mosquitoes
biting indoors in the daytime were Ae. aegypti. The survey for dengue vectors in five
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geographical zones of Thailand demonstrated that Ae. aegypti predominates in all
areas whereas Ae. albopictus is restricted to the southern part of the country. As
previously reported, water jars are the most important breeding sites of Ae. aegypti,
while broken cans and plastic containers are the preferred breeding habitats of Ae.
albopictus (Chareonviriyaphap et al. 2003). An ecological survey of dengue vectors
carried out in central Laos during the year 2000 reported that Ae. aegypti is dominant
among 7 species collected. The key habitats are water jars, cement water tanks, drums
and discarded containers, while containers containing Mesocyclops do not have Aedes
larvae (Tsuda et al. 2002).
Longitudinal studies on dengue vectors were conducted in Thailand. Seasonal
distribution of Aedes larvae in Eastern Thailand reported that even when the larvae are
less abundant during the dry season, every part of the studied villages have some of
them (Strickman and Kittayapong 2002). This study suggests that vector control in
Southeast Asia should concentrate in schools or areas with greatest abundance based
on the calculation of larval indices. Another study in Eastern Thailand also showed
that breeding containers with high larval nutrients produce large numbers of pupae
and large-sized mosquitoes. An estimate of the number of females per house was
above the threshold for increasing transmission in all months except from December
to February. The number of pupae per house and local temperature were used to
calculate transmission risk using Focks’ model. Results indicated that the risk is
greatest in the months of May and June (Strickman and Kittayapong 2003). Studies on
the population dynamics of Ae. aegypti in Thailand showed that temperature, but not
rainfall, is correlated with female abundance. In addition, high temperature may
increase age distribution of young adults and frequent blood feeding due to rapid
reduction of energy reserves (Scott et al. 2000b). Blood-feeding behaviour of
mosquito vectors is important for the understanding of dengue transmission. Multiple
blood feeding and micro-movement to obtain blood sources have been confirmed for
Ae. aegypti using PCR-based identification of human-blood meals (Chow-Shaffer et
al. 2000). In Thailand, it was found that 65% of Ae. aegypti feed twice on the same
day (Scott et al. 2000a). Mark–release–recapture studies in Thailand indicated that the
survival rate of Ae. aegypti was age-dependent. Traditional linear regression analysis
showed that the survival rate of older females was significantly greater than that of
younger ones whereas the more sensitive non-linear regression analysis could not
detect differences in the survival rate of both age cohorts in Thailand (Harrington et
al. 2001).
Field studies in Thailand concerning the Wolbachia endosymbiont of Ae.
albopictus have been reported. These bacteria are found to infect several species of
Southeast-Asian mosquitoes (Kittayapong et al. 2000) and might be used in genetic
control through cytoplasmic-incompatibility-induced population replacement. In
nature, Aedes albopictus is double-infected with these bacteria whereas Ae. aegypti
has never been reported to be infected. Recently, stable infections of Wolbachia in Ae.
aegypti have been successfully obtained and it was found that these transinfected lines
do not exhibit differences in fitness when compared to naturally uninfected
populations (Ruang-areerate et al., unpubl. data). Cross-mating between Wolbachia-
infected and uninfected Ae. albopictus has shown that Wolbachia-mediated
cytoplasmic incompatibility in field-caught and laboratory-reared old-aged Aedes
albopictus is very strong (Kittayapong et al. 2002). Maternal transmission and field
prevalence of 100% in natural populations of Wolbachia-double-infected Ae.
albopictus in Thailand support the potential application of these bacteria as gene-
driving mechanism (Kittayapong et al. 2002; Kittayapong, Baimai and O'Neill 2002).
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Population genetics, vector competence and dengue transmission
DF and DHF have long been reported as the most common urban diseases in the
Southeast-Asian region since the 1950s, before spreading worldwide. All four
serotypes of dengue viruses co-circulate in this region (Gubler and Kuno 1997). A
study showing the spread of DHF by travellers from East Timor to Townsville,
Australia evidenced the importance of humans as vehicles for disease spreading (Hills
et al. 2000). There has also been a report on the high risk of DHF in the area where
foreigners work for petroleum companies in Indonesia, subsequently returning to their
home countries (Mangara et al. 2000). Seroprevalence of dengue virus among German
overseas aid workers was found to be 6.4% (43/670), and of these 43, the highest
seroprevalence (19.4%) was detected in those returning from Thailand (Eisenhut,
Schwarz and Hegenscheid 1999). As reported from the study by Harrington et al. (in
press) in Thailand, the human-movement factor may perhaps be more important in the
spreading dynamics of the disease than the dispersal and flight range of Aedes vectors.
This idea is supported by the recent estimates of population-genetic organization and
gene flow in Ae. aegypti using microsatellite markers (Huber et al. 2004). This study
showed that there is less genetic differentiation between mosquito populations from
Vietnam (Ho Chi Minh City) and Cambodia (Phnom Penh) than between either of
them and Thai populations, suggesting that passive migration through human
transportation is the major cause of vector spreading.
Genetic structure of Ae. aegypti was studied in Vietnam in relation to vectorial
competence and resistance to insecticides (Huber et al. 2003). Estimation of
population-genetic organization and gene flow showed that ecological disturbance
through urbanization, which had direct impact on sanitation, has a direct impact on the
vectorial system. The relationship between genetic differentiation and vector
competence for dengue-2 virus has been reported (Huber et al. 2002a). Genetic
variations in Ho Chi Minh City and its outskirts were studied using starch-gel
electrophoresis and microsatellite markers. Results showed that genetic differentiation
is lower in the city when compared to its outskirts, depending on the abundance of
breeding sites and human hosts as well as the insecticidal control during dengue
outbreaks (Tran et al. 1999; Huber et al. 2002b). Moreover, seasonal and
environmental factors also had an effect on the genetic structure of Aedes vector
populations (Huber et al. 2002c). In Thailand, genetic differentiation was confirmed
in Ae. aegypti samples collected from different subdistricts. Results may be related to
insecticide treatment in these areas (Mousson et al. 2002). In conclusion, further
studies on genetic variations of vector populations are required to provide further
insights into the understanding of disease epidemiology.
A study on the major mosquito fauna in the forested area undergoing development
of an oil palm plantation in Sarawak, Malaysia showed the reduction of the species
composition of malaria vectors and the risk of malaria transmission but, on the other
hand, an increase of dengue vectors and the risk of dengue transmission (Chang et al.
1997). Evidences for dengue infection in both vector species, i.e., Ae. aegypti and Ae.
albopictus, were reported from Samui Island, Thailand (Thavara et al. 1996).
Transovarial transmission of dengue viruses was reported in Singapore. Males of both
species were found positive with dengue viruses using a type-specific PCR technique.
The serotypes were checked and the results showed a negative correlation between
DEN-1 and DEN-4. DEN-1 was higher than DEN-4 in Ae. aegypti while the opposite
was detected in Ae. albopictus (Kow, Koon and Yin 2001).
A study in Selangor, Malaysia revealed a positive correlation between a dengue
outbreak and rainfall pattern, which increased the number of breeding habitats of
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Aedes vectors (Li et al. 1985). Biological and entomological parameters related to a
seasonal pattern of dengue using a mathematical model reported that the strongest
influence on the seasonality and pattern of dengue transmission is the duration of
infectiousness of the host, vector mortality and biting rates (Bartley, Donnelly and
Garnett 2002). Susceptibility to dengue viruses among Ae. aegypti mosquitoes
collected in different seasons of the year showed no seasonal correlation, even though
a seasonal pattern of dengue transmission was observed in Thailand. It was suggested
that characteristics of the virus, vector density and frequency of host–vector contact
should be considered instead (Thongrungkiat et al. 2003).
Distribution of dengue and Japanese encephalitis among children in rural and sub-
rural areas in Thailand has been studied. The results showed that most transmission
occurs in residential environments and within a young age group (3-8 years old),
which has a significantly higher risk of infection than older children (Strickman et al.
2000). An epidemiological study of DHF in Thailand suggested that vector control
activities should concentrate on areas and populations at higher risk (Barbazan,
Yoksan and Gonzalez 2002). Integration of geography and pathology for DHF is
required to understand the complex epidemiology of the disease, which depends on a
variety of factors (Menard 2003). Recently, the spatial and temporal studies of DHF
incidences in all 73 provinces of Thailand (1983-1995) could discriminate between
seasonal and non-seasonal transmission. The spatial-temporal dynamics of DHF
incidence using the data set of 850,000 infections in Thailand from 1983 to 1997
showed that the disease occurred first in Bangkok, the largest city and capital of
Thailand, and then moved radially at the speed of 148 km per month. This finding
provided a crucial piece of information, namely that the permanent viral reservoir is to
be found in large cities; these should therefore be the targets for long-term control of
DHF (Cummings et al. 2004).
Dengue vector control
Up until the present, there has been no promising solution for sustainable control of
dengue vectors. The trend for dengue vector control in this region has shifted from
relying solely on insecticides to biological control, source reduction and
environmental management through community participation (Gubler and Kuno
1997). Several countries in the region have recently carried out integration of vector
control approaches.
In Vietnam, a survey for Mesocyclops, Micronecta and fish as biological control
agents demonstrated that a large number of breeding containers already contained
Mesocyclops and the presence of both Mesocyclops and Micronecta provided some
level of control (Nam et al. 2000). A few years later, a successful dengue vector
control programme was initiated in three provinces of Northern Vietnam with an
application of the biological control agent, Mesocyclops spp., and clean-up campaigns
through community participation (Kay et al. 2002). Recently, prolonged efficacy in
controlling Aedes larvae in water containers by a combination of Bacillus
thuringiensis israelensis (Bti) and copepods was evaluated in Thailand
(Kosiyachinda, Bhumiratana and Kittayapong 2003). The enhancement of control
activities was observed when rice grains were used as supplementary food for
copepods.
From Malaysia, there are few reports on high efficiency of a combination of
biological and chemical insecticides (Seleena, Lee and Chiang 2001; Sulaiman et al.
1997; 1999; 2000). The details of dengue vector control and dengue situation in
Malaysia were discussed by Poovaneswari (1993), Tham (1993), and Yap et al.
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(1994). Singapore has a well-established system for dengue vector surveillance and
control. The control strategy integrates case detection, source reduction, health
education and law enforcement (Wang 1994). Application of an autocidal ovitrap as a
vector control measure against Ae. aegypti was developed in Singapore since 1977 by
Lok, Kiat and Koh (1977). Recently, a mixture of Bti (Vectobac 12 AS) and
pirimiphos-methyl (Actellic 50 EC) applied through thermal fogging at various
heights and distances was tested as an efficient approach for simultaneously
controlling both larvae and adults of Ae. aegypti in Singapore. However, there is a
report from Central Java, Indonesia of an unsuccessful vector control trial when
Toxorhynchites was used as a biological control agent (Annis et al. 1990).
In Thailand, Gratz (1993) pointed out that dengue vector control, as in most other
countries, had made little use of the methodologies arising from research. After a long
routine application of insecticides as a vector control measure, the trend in Thailand
was recently geared toward environmental protection (Chunsuttiwat and Wasakarawa
1994). Several studies were conducted in Thailand regarding the use of repellents,
physical and biological control agents. Three types of screen covers were developed
for preventing vectors from breeding in water jars, which were the most common and
important breeding site of Ae. aegypti in Southeast Asia (Kittayapong and Strickman
1993). Thanaka (Limonia acidissima) and DEET (di-methyl benzamide) mixture were
evaluated for their efficacy as repellents against both Anopheles and Aedes vectors in
Thailand. Almost complete protection up to 6 hours was obtained with a combination
of 20% thanaka and 0.5% permethrin. Laboratory bioassays with Ae. aegypti
indicated that the combination could extend protection from exposure up to 10 hours
(Lindsay et al. 1998). The Thai strain of mosquito densoviruses infecting both Ae.
aegypti and Ae. albopictus was reported to be efficient in killing Aedes larvae and
could be developed as a biological control agent (Kittayapong, Baisley and O'Neill
1999). Evaluation of Bti tablets and temephos ZG were conducted in Thailand.
Results showed that at the dosage of 0.37 g per 50 l of water could provide control
activities between 90 and112 days while a new formulation of zeolite granules (ZG)
of temephos (1%) at the operation rate of 5 g per 50 l of water yielded 100% control
for more than 6 months. Both larvicides increased clarity of the water with no
unpleasant odour (Mulla et al. 2004).
As synthetic pyrethroids are still used in the public-health system to control vector-
borne diseases, the development of insecticide resistance in vectors remains an issue
of concern. Some information on vector resistance to pesticides in Thailand was
reported by Chareonviriyahpap, Aum-aung and Ratanatham (1999). Brengues et al.
(2003) reported the presence of permethrin resistance in a few localities in Southeast-
Asian countries, i.e., Indonesia, Thailand and Vietnam. The evidence of both DDT
and pyrethroid resistance in Indonesia and Vietnam suggested the presence of a
knock-down resistant kdr-type mechanism.
Social science and socio-economics related to dengue
It has been reported that health education had an effect on the outcome of a DHF
vector control programme in an urban area of Northern Thailand by reducing the
Breteau index to about half (from 241 to 126) (Swaddiwudhipong et al. 1992). A
recent study on climatic and social risk factors for Aedes infestation in rural Thailand
reported that factors such as availability of public water wells, existence of transport
services and proportion of tin houses, were positively associated with larval indices
(Nagao et al. 2003). Socio-economic factors, i.e., per capita gross provincial product
(GPPpc) and health-care resources in relation to geographic distribution of malaria
119
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and dengue in Thailand, were also examined. It was recommended from the study that
this approach be used for considering resource utilization in integrated control of both
diseases (Indaratna et al. 1998). In Northern Thailand, a KAP (Knowledge, Attitudes
and Practices) survey reported that 67% of people in the study area had knowledge of
dengue, significantly different with respect to age, sex and occupation. Young
students had a higher level of knowledge of dengue when compared to older
housewives and unemployed persons. In addition, people with knowledge of dengue
reported more frequently the use of preventive measures against vectors (Van
Benthem et al. 2002). Another KAP survey was carried out among the caretakers of
DHF in primary schools in one province in central Thailand. The majority of people
in this study area were mothers with primary-school education. Results indicated that
they need more understanding of the disease. In general, the caretakers whose
children had DHF had higher response in prevention, control and treatment than the
group who had healthy children (Kittigul et al. 2003). Both studies demonstrated the
importance of educational campaigns to obtain community participation in DHF
control. A KAP survey recently reported from Malaysia reflected that most people
have a high level of knowledge about dengue and vector control (Hairi et al. 2003). In
addition, both were significantly correlated with the attitude towards Aedes control.
However, there was no correlation between the level of knowledge and the vector
control practice, which implied that a high level of knowledge did not necessarily lead
to good practice. Therefore, differences in social factors and cultures in each country
need to be considered for planning educational programmes.
Conclusions
In conclusion, long-term vector control approaches include source reduction and
environmental management, chemical and microbial larvicides, and personal
protection using household insecticide products and repellents. Space spray, both
thermal fogging and ultra-low volume, is normally used as short-term control
measure, especially during disease epidemics. Practical vector control approaches in
Southeast Asia rely on persistent efforts by the government sectors as well as
communities themselves leading to variations in the success of vector control
programmes. Planning and decision-making for resource utilization for malaria and
dengue control at both national and regional levels could be more efficient through
co-analysis of the disease-epidemic patterns and utilization of health-care resources.
Due to the fact that both malaria and dengue vaccines are still under development and
the current vector control programmes hardly provide a long-lasting effect,
transgenesis-based development of refractory strains of mosquito vectors that are
refractory to disease pathogens may offer a new alternative for disease control, which
could be successful regardless of the level of community participation. However,
genetically modified mosquitoes need to be very efficient in spreading themselves and
competing with natural vector populations. In addition, acceptance of the strategy by
the community itself will be an important issue. Therefore, the biology and ecology of
vectors as well as social and socio-economic factors in different geographic regions
need to be investigated in more detail to confirm the possibility of application.
120
Kittayapong
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